Ok, so I’ve done a bunch of testing and so far here is what I learned. The power linear regulator works great so far with only a 5% or so drop in load regulation at full load. Tested it at a few voltages throughout the range. I ran into a bunch of problems in my layout so I’m probably going to re-do the board at some point once I’m sure I have all the kinks out. The two op amps for the voltage and current regulation were laid out inverse (my own fault for not triple checking) and so was the high side current sense amp. Lessons learned as this was one of those projects that has thus far spanned months of downtime as I was away for work or on other projects etc.
One question I did have at this point is if anyone has a good suggestion on a better on board power supply is. What I realized when I was testing the fan outputs is that the current 7805 is definitely not going to cut it!
They run fine at low speeds as expected but above 30% or so their current draw becomes such that it starts to drag the 5V rail down as the 7805BG over heats. I know this is due to the fact that I’m drawing upwards of half an amp overall (170Ma per fan X 2 fans plus whatever the other digital components are drawing) and my input voltage is 19.8 volts. That puts things at just over 5 watts of power the little Dpak-3 has to dissipate, NOT GOOD! My first thought was to add a second regulator in a larger package just for the fans but now I’m thinking a small buck converter for the whole 5V rail instead. My concern with that is the noise of a switcher as I’m trying to make this as low noise as I can with it being a fully linear power supply and all.
Anyone have more experience with the overall output noise on switchers than me? I’m also aware that I can use a linear regulator as a post regulator to smooth things out as well. Say, buck the voltage down to 7.5-8V then a 5V regulator from there but that’s starting to increase my parts count and board space which I’d like to keep down as well. As well as my limited working experience with working with switch mode parts.